Hello.

This is patch attempt to target the discussion about sub-optimal generation
of DO loops with abs(step) == 1.

Originally generated code:

    D.3428 = (*array)[0];
    D.3429 = (*array)[1];
    i = D.3428;
    if (i <= D.3429)
      {
        while (1)
          {
            {
              logical(kind=4) D.3432;

              (*block)[(integer(kind=8)) i + -1] = (*block)[(integer(kind=8)) i + -1] + 10;
              L.1:;
              D.3432 = i == D.3429;
              i = i + 1;
              if (D.3432) goto L.2;
            }
          }
      }
    L.2:;

Suggested:

    D.3428 = (*array)[0];
    D.3429 = (*array)[1];
    i = D.3428;
    while (1)
      {
        {
          logical(kind=4) D.3432;

          D.3432 = i > D.3429;
          if (D.3432) goto L.2;
          (*block)[(integer(kind=8)) i + -1] = (*block)[(integer(kind=8)) i + -1] + 10;
          L.1:;
          i = i + 1;
        }
      }
    L.2:;

I would like to enable the behavior be default, patch does that starting from -O2.

Apart from that, I've also added a new warning (candidate name: Wundefined-do-loop), which warns about cases that
would lead to an infinite loop (because i > HUGE(i) == false):

Warning: DO loop at (1) is infinite as it iterates to MIN_INT [-Wundefined-do-loop]
/tmp/loop2.f90:15:19:

   do i = 0, huge(i)
                   1

Patch hasn't been properly tested, I've just tried to run all do_*.[fF]90 tests.

Thoughts?
Martin